Modeling and experiments of X-ray ablation of National Ignition Facility first wall materials

This paper discusses results of modeling and experiments on the x-ray response of selected materials relevant to the NIF target chamber design. X-ray energy deposition occurs in such small characteristic depths (on the order of a micron) that thermal conduction and hydrodynamic motion significantly affect the material response, even during the typical 10-ns pulses. The finite-difference ablation model integrates four separate processes: x-ray energy deposition, heat conduction, hydrodynamics, and surface vaporization. Experiments have been conducted at the Nova laser facility in Livermore on the response of various materials to NIF-relevant x-ray fluences. Samples of fused silica, silicon nitride, boron carbide, boron, silicon carbide, carbon, aluminum oxide, and aluminum were tested. The response was diagnosed using post-shot examinations of the surfaces with scanning electron microscope (SEM) and atomic force microscope (AFM) instruments. On the basis of these observations, judgments were made about the dominant removal mechanisms for each material. The relative importances of these processes were also investigated with the x-ray response model.